272 MORRIS M. LEIGH TON 



Comparison with the Illinoian area to the west. — The features of 

 the Illinoian area to the west, taken as a whole, are in considerable 

 contrast to those just mentioned for the Belvidere lobe. The ero- 

 sional topography west of Rock River is largely an inheritance of 

 preglacial erosion, but locally there are morainic belts and post- 

 glacial gorges whose degree of erosion appears greater than that of 

 the Belvidere lobe. The oxidation of the drift beyond the Belvidere 

 lobe is stronger than that of the Belvidere drift, the color ranging 

 from brown to rusty brown; the till is more compact and the upper 

 part has a greater concentration of residual pebbles ; the mantle of 

 loess is thicker; the loess rests on the tiU unconformably, that is to 

 say, the till beneath the calcareous and fossiliferous loess is oxidized 

 and leached, whereas in the few instances in the Belvidere lobe 

 where calcareous loess was penetrated by the auger it was found to 

 rest on calcareous till containing limestone pebbles; gumbotil was 

 found in a score of places in the lUinoian area, whereas no occurrence 

 of this distinctive material is known in the Belvidere lobe except 

 possibly at two points where the auger penetrated a gumbo-like 

 till after passing through relatively fresh drift. (See Fig. i for the 

 distribution of the exposures and borings which show this and 

 other significant data.) Another very characteristic phenomenon 

 of the extra-Belvidere drift is the occurrence of an old loess-like 

 clay, if to 4 feet thick, which is non-calcareous, compacted, oxidized 

 to a brownish color, contains manganese pellets, and wherever 

 found has a stratigraphic position between the yellow loess above 

 and the till below. In some places this old loess-like clay contains 

 dark organic matter as if it had formerly marked a soil horizon and 

 supported vegetation. This was not found within the B elvidere lobe. 

 One hundred and fifty borings in the extra-Belvidere drift showed 

 an average of 3.9 feet of leached soil and loess and 4.2 feet of leached 

 till over calcareous till with limestone pebbles, making a total of 

 8.1 feet of leached material as compared with 4 feet on the Belvidere 

 lobe. (See Table I and also Fig. 3.) Since loess leaches more 

 rapidly than till, it is also to be noted that the figures for the lUi- 

 noian area include a greater proportion of till than for the Belvidere 

 lobe, and furthermore that 23 per cent of the borings failed to reach 

 the calcareous zone in the lUinoian area, whereas only 6 percent 



